Interest to microlens array systems arose in the beginning of 60's, when it was found that the optical systems assembled from microlens arrays make it possible to achieve a number of optical characteristics, such as a large ratio of an input optical aperture to the depth of the system, unattainable in conventional photographic cameras. This made it possible to create optical systems of high compactness.
In this connection, reference can be made to U.S. Pat. No. 3,447,438 issued in 1969 to H. Kaufer, et al. and U.S. Pat. No. 3,605,593 issued in 1971 and reissued in 1974 as U.S. Pat. No. Re. 28,162 to R. Anderson. These patents for the first time have explicitly formulated possibility of reducing the length of the system in the direction of the optical axis and thus improving the system's compactness due to the use of lens arrays.
Only much later, i.e., in 90's and in the beginning of 2000's, lens and microlens arrays have found application in designs relating to photolithography, image-sensing, image digitization, etc. The development history of contact-type optical systems based on the use of microlens arrays is described in aforementioned U.S. Pat. application Ser. No. 11/075253.
In fact, the main optical approach used practically in all previous patents of the aforementioned category has been formulated in a much earlier article of R. Anderson issued under the title “Close-up Imaging of Documents and Displays with Lens Arrays” in Applied Optics, Vol. 18, No. 4, pp. 477–484.
Anderson's objective was creation of a photo camera for registering images of an oscilloscopic tube. At a design stage, however, it was understood that the system developed by the author could find a much wider practical application. As it is stated in the article, in many close-up imaging systems, e.g., in document copier machines, the housing or structural body of the imaging system extends over the entire length of the optical path from the object to the image. The shortest normal optical path length of such a system is the one associated with 1× magnification, where the image and the object are each at a distance of four focal lengths from the lens, and the total optical path length is equal to eight focal lengths plus some distance that corresponds to the total thickness of optical elements. This is because an intermediate image has to be formed between the final image plane and the object. Accordingly, those systems are several times as long as a general-purpose distant object camera having a lens of the same focal length, where the camera body extends only over an image distance of about one focal length. R. Anderson showed that by assembling the optical system from microlens arrays it was possible to significantly shorten the total length of the optical system in the optical axis direction. This short length is one of the optical properties of two parallel arrays of simple lenses arranged in rows and columns. The new optical system may have wider use than oscilloscope photography, however, unlike conventional optics, the length of the system does not increase with an increase in the size of the object field to be covered. Imaging of larger objects such as copier machine documents or computer peripheral displays requires larger lens arrays, but does not require a longer optical system.
In principle, a lens-array optical system used, e.g., for very close-up photography of large object, would be photographing simultaneously small sections of the large object with an array of many cameras arranged in rows and columns, where each camera has only a limited field of coverage. The resulting separate photographs obtained in such a process could be assembled together, while each lens of the array functions as an objective of each individual camera.
One may think that it would be much easier to combine the film-backs of all the cameras into a single larger film-back faced by an array of lenses in a common lens-board. However, each coaxial lens set is intended for inverting and reverting its portion of the image relative to the object, while the combined inversion or reversion of the composite image will not produce a real image. Where the adjacent inverted images meet or overlap a distortion occurs since in the overlapped areas images are not of the same object points.
In his article, R. Anderson analyzes the sources of the aforementioned overlapping on the edges of the individual images and offers a method for attenuating the overlapping. In conclusion of his work, R. Anderson formulates the following dominant principles required for the formation of correct images: 1) the image plane must coincide with the plane of coincidence, i.e., the plane where edges of adjacent images coincide and thus overlap each other; 2) the lens-pair magnification must equal the composite image magnification; and, 3) in symmetrical systems, the object distance (i.e., the distance from the object to the first lens array) equals the coincidence plane distance (i.e., the distance from the second lens array to the coincidence plane that coincides with the focal plane of the second lens array). In order to provide short length and large image area combined with good brightness, contrast, and resolution, the parallel lens arrays should have dimensions which satisfy the requirements of Items 1)–3).
Based on the principles formulated in the aforementioned article, R. Anderson developed an optical apparatus with a short longitudinal length that included a pair of microlens arrays (that are called by the inventor as mosaics) of optical imaging elements. This apparatus is disclosed in Anderson's U.S. Pat. No. Re. 28,162 issued in 1974 that, in addition to the features described in the article, also includes an adjustable stop for each microlens for limiting the light-passing apertures as a measure of restricting partial image overlapping or for increasing the apertures in order to join the boundaries of the adjacent images and thus to form a large continuous image.
A similar problem was solved in aforementioned U.S. Pat. No. 3,447,438 of Kaufer et al. that relates to an optical system having at least two lenticular screens. In fact, each screen is a lens array. Furthermore, the system is provided with a diaphragm array arranged between the lens arrays and having each diaphragm opening coaxial with respective coaxial lenses. However, the aperture-adjusting mechanism of Kaufman is different from that of R. Anderson, and adjustment of the apertures is carried out by performing relative movements of two plates with overlapping openings that determine a degree of opening of the diaphragms.
Two last-mentioned patents have demonstrated all the advantages resulting from application of optical lens arrays for creating compact optical systems. However, it is understood that the principle of mechanical adjustment of apertures on individual lenses, even though combined into an array, is inapplicable to microlenses of microlens arrays, where lenses have characteristic dimensions in the range of hundred microns or less.
Optical microlens assemblies with dimensions of several ten to several hundred microns were introduced into practical use in 1990's. In these systems, the optical-signal receiving elements were implemented in the form of CCD or CMOS arrays with pixel dimensions equal to or smaller than the size of the microlens. The aforementioned pixels had dimensions from several microns to several ten microns. It is obvious that in such systems the problem of eliminating overlapping between the adjacent images created by neighboring microlenses or joining spaced adjacent images into a single big image becomes even more exaggerated. This is because the mechanical aperture-adjustment mechanisms for adjusting individual lens apertures become practically impossible in view of microscopic dimensions in diametrical and thickness directions.
A trivial attempt of solving the above problem is described in aforementioned U.S. Pat. No. 6,057,538 of J. Clarke. It was suggested to reduce overlapping of adjacent images by reducing the microlens diameters and by masking the spaces between the microlenses with light-blocking coating in order to restrict the aperture of the microlens opening and thus to eliminate overlapping. An additional measure for preventing undesired image overlapping is the use of a matrix of vertical walls for limiting lateral illumination of correct images. Although such measures as reducing the size of the microlenses, masking, or shielding the side illumination produce some effect, this is achieved at the expense of light efficiency that is diminished.
However, none of the references mentioned above offer a method or system that allows substantially complete elimination of overlapping of individual images produced by individual lenses or microlenses. For example, according to the principle described in the aforementioned article of R. Anderson, minimization of overlapping is carried out exclusively by selecting appropriate distances between the planes of lenses and the image plane. The function of diaphragms in such a system was fulfilled by apertures of microlenses themselves, and spaces between them were masked. The system described in the aforementioned patents of R. Anderson and H. Kaufer contained diaphragms located practically in the planes of lens arrays. All these diaphragms had cross-sectional shapes that could not completely eliminate at least partial overlapping of adjacent images produced by adjacent lenses. In some systems, the apertures of these diaphragms could be mechanically adjusted. However, the principle of such adjustments is absolutely inapplicable at the microlens assembly level. Inevitable overlapping did not allow obtaining of non-distorted images.
The applicants have solved the above problems by providing a thin monolithic image sensor disclosed in aforementioned U.S. patent application Ser. No. 11/079549. The sensor is comprised of a laminated solid package composed essentially of an optical layer and an image-receiving layer placed on the top of the optical layer. The optical layer also comprises a laminated structure composed of at least an optical microlens-array sublayer and an aperture-array sublayer. The image-receiving layer is a thin flat CCD/CMOS structure that may have a thickness of less than 1 mm. The image digitized by the CCD/CMOS structure of the sensor can be transmitted from the output of the image-receiving layer to a CPU for subsequent processing and, if necessary, for displaying. A distinguishing feature of the aforementioned sensor is that the entire sensor, along with a light source, has a monolithic structure, and that the diaphragm arrays are located in planes different from the plane of the microlens array and provide the most efficient protection against overlapping of images produced by neighboring microlenses.
Although the above sensor is capable of producing a non-distorted image with substantially complete elimination of overlapping of individual images produced by individual lenses or microlenses, the use of the aforementioned sensor is limited to specific applications where the sensor is to be in contact with the object to be reproduced. In other words, the sensor of U.S. patent application Ser. No. 11/079549 cannot reproduce an image of an object located at infinity.
An attempt of imaging a remote object with the use of a microlens array system is described in a series of article and patents by R. Volkel, et al. (see, for example, “Microoptical telescope eye” by R. Volkel, et al., 7 Feb. 2005, Vol. 13, No. 3, OPTICS EXPRESS 889). In his work, R. Volkel refers to GB Patent No. 541753 of D. Gabor published as early as 1941. In his patent, D. Gabor for the first time introduced a concept of a so-called superlens that is now known as the Gabor superlens. The Gabor superlens comprises an imaging system of two microlens arrays. Respective microlenses of both microlens arrays have parallel optical axes, but the pitches of the microlenses in both arrays are different and neither an integral multiple of the other. The separation of the arrays is equal to the algebraic sum of their local lengths, if both the arrays are transmitting or, if one of the arrays is reflecting or backed by a plane reflector, is equal to the algebraic sum of twice the focal length of the reflecting microlenses and the focal length of the others. D. Gabor showed that the arrays are equivalent to “superlenses” causing parallel incident light to emerge in parallel or nearly parallel bundles which unite to form “superfocal” lines much smaller in number than the number of microlenses in the arrays. Under certain condition or relations between the focal distance and pitches of microlenses in both arrays only one “superfocus” is formed. In his patent, D. Gabor considered such a condition and showed how to register all individual images produced by a plurality of microlenses into a single image.
Much later R. Volkel, et al. used the Gabor's superlenses for creating a single image from a plurality of individual images converged into a common point. For improving quality of images, R. Volkel, et al. introduced diaphragms in the plane of microlenses and offered to eliminate or reduce image distortion due to the use of anamorphotic microlenses, e.g., elliptical microlenses.
Similar to the Gabor's system, the Volkel, et al. system comprises a number of microlens arrays arranged in series with sequentially reduced pitches between the adjacent microlenses. The microlenses that are aligned, i.e., arranged in the same microlens channel, represents an elemental optical system for building an elemental image of a remotely located object. It is understood that the number of such elemental images is equal to the number of microlens channels, i.e. to the number of microlenses in each array. The optical axes of the aforementioned elemental microlens channels converge in a single point located in the image plane. In other words, the creation of the final image of the remote object is reduced to interposition of all elemental images onto each other in the image plane. A main disadvantage of the aforementioned approach is that each microlens channel, especially the microlens channels on the peripheries of the arrays, form optical systems composed of microlens sequence with the planes of the lens not perpendicular to the planes of the lenses, i.e., to the planes of the arrays. As a result, all microlens channels (except for the central one), and especially peripheral channels, are subject to violation of paraxiality of rays. This means that the individual images created by the peripheral microlenses will be distorted. It is understood that dimensions of the final image and the total aperture of the arrayed system are in contradiction, and this contradiction is fundamental. This contradiction significantly limits design capabilities for practically acceptable systems. As mentioned above, an improvement introduced by R. Volkel into the Gabor's system is the use of diaphragms that, similar to the Anderson's system, restrict the microlens apertures and prevent the edge overlapping. Another improvement is a modified shape of the microlenses by introduction of anamorphotic lenses to compensate distortions, especially on the edges. Nevertheless, in spite of the fact the Volkel's, et al. system was the first microlens system for imaging a remote object, this system had significant limitations with regard to the field of view (FOV) and could not be implemented with fields of view exceeding, e.g., 10°, i.e., could be used essentially only in telescopic optical systems. This is a significant drawback that limited practical applications of the system.